High elongation at break means the elongation at break of the cord is at least 5%. It is well know that productivity of the cord will increase with the lay length increasing. However, a high lay length of the cord will cause the loss of elongation at break. Once a steel cord is formed with a higher lay length, its elongation at break will be lower. Generally the cord and the strand are twisted in same direction to get a high elongation at break.
In solid mechanics, E-modulus is a measure of the stiffness of a material in the elastic region. It is defined as the ratio of the stress over the strain in the range of stress in which Hooke's Law holds. This can be experimentally determined from the slope of a stress-strain curve created during tensile tests conducted on a sample of the material. To linear materials such as steel, E-modulus is essentially constant over a range of strains.
The stiffness of a cord is of principal importance to keep the tire diameter stable in high speed, so the E-modulus is often one of the primary properties considered when selecting a cord structure. As the E-modulus is higher, the steel cord is stiffer.
U.S. Pat. No. 5,661,966 discloses a steel cord with high lay length and high elongation at break. The steel cord comprising plurality strands has an elongation of more than 5%. The lay length of the cord is 8 to 15 times the diameter of the cord. The filaments of the strand are wavy preformed with a pitch smaller than the lay length of the strands before twisting together. Generally the loss of elongation at break when using long lay length can be compensated by preforming filaments. Although the elongation at break and lay length is high, there is a problem that the E-modulus is small and the steel cord is not very stiff.
3×7 high elongation at break steel cord is a popular cord in the market to reinforce the tire. But there is a drawback that the cord is formed with low lay length. So it is produced with a high cost. The breaking load of the cord is not very high. Also the E-modulus is low so that the cord is not very stiff.